1. The Field of the Invention
The present invention relates to rocket motors, especially for gun-launched projectiles, having insensitive munitions systems.
2. Description of the Related Art
Many launchable projectiles, rockets, and rocket stages comprise a forward end, including guidance and munitions, and an aft end rocket motor. These two elements can be formed together, with a common outer case, or they can be separately formed and subsequently joined together. This joining can occur immediately prior to use, in which case the two elements may be separately stored, or the elements can be joined together for storage purposes and be ready for immediate use.
During prelaunch storage, when a rocket motor is ignited inadvertently by external heating, such as a spilled fuel fire, the rocket motor may become propulsive before being properly aimed. When inadvertent ignition is caused by fragment impact that produces unplanned nozzle outlets, the motor may become wildly propulsive in undesired directions. When such events produce unplanned increases of propellant burning surface area, excessive pressurization may increase the hazard to nearby personnel and property. In light of these dangers, many of today""s weapon systems must satisfy certain insensitive munitions (IM) requirements focused on safe storage capabilities.
One way that rocket motors meet IM requirements is by venting the internal pressure caused through inadvertent ignition of the propellant by discharging either the forward or aft closure of the case cylinder. This allows the propellant to burn through a now open end that is relatively large compared to the nozzle throat without generating substantial thrust in any direction and without the threat of the rocket motor exploding and spraying burning propellant and metal case cylinder fragments in numerous directions.
The prior art teaches the use of dual paths for load transfer between features of either closure or between the closure and the motor case cylinder. One such load path may be sized to accommodate relatively small loads that might be experienced during transportation and handling prior to gun launch, and the other to accommodate much larger loads encountered during launch or during rocket motor operation. Focusing on shells that may or may not include rocket motors, U.S. Pat. No. 4,557,198 discloses shear pins or locking rings arranged for arming the high capability load path by axial acceleration during normal launch and disarming the low capability load path. Boissiere, in U.S. Pat. No. 5,337,672 (1994), teaches arming of the high capability load path and disarming the low capability load path by gas pressures produced by the round itself. Dolan, in U.S. Pat. No. 4,597,261 (1986), Panella in U.S. Pat. No. 3,887,991 (1975), Tate in U.S. Pat. No. 5,036,658, Koontz in U.S. Pat. No. 5,155,298 (1992), Ellingsen in U.S. Pat. No. 5,311,820, and Cherry, in Statutory Invention Registration H1144 disclose the use of thermally activated devices of similar intent. Further, Malamas, in U.S. Pat. No. 4,991,513, discloses use of a vent system that is closed by spin-up at launch. Singer et al., in U.S. Pat. No. 6,094,906, discloses a more recent approach for generating a vent path for IM protection.
The safe expulsion of either closure can also be accomplished through the use of a low shear retaining meansxe2x80x94positioned between components of the closure or between the closure and the rocket motor case cylinderxe2x80x94and a high capability load path that is disarmed until subjected to gun pressure. Should the propellant be inadvertently ignited, the low shear retention means will shear under relatively low internal pressure and allow the entire closure, or a portion thereof, to disengage from the case cylinder. Thus, the internal pressure induced by inadvertent ignition will vent without the dangers associated with premature propulsion or explosion.
One problem associated with many of these conventional IM systems is that they do not pass slow cook-off tests. For many conventional IM systems, heating at relatively slow rates of about 6xc2x0 F./hr causes the entire propellant to combust substantially instantaneously prior to activation of the IM systems, producing excess gas which the IM systems are not equipped to handle and safely expel.
In the case of gun-launched missiles, other design criteria that should be taken into consideration pertain to the thermal expansion characteristic of composite solid propellants. Composite solid propellants are one of two general types of solid propellants for rockets. In composite solid propellants, the fuel and oxidizer particles are bound together by a cured rubber matrix. Composite propellants have burning surface areas that may be readily controlled by adjusting the shape of the solid material and the burn rate features of the formulation. The other type of solid propellant is compressed powders. For compressed powders, virtually the entire cumulative surface area of all the particles is available for combustion immediately upon ignition. During the burn of a compressed powder propellant, vastly higher operating pressures prevail than during burn of a like quantity of composite propellants. It follows that compressed powder propellants are generally used only where the gun barrel can withstand the high pressures. When the propellant is to burn after the rocket leaves the gun, generally a composite propellant is chosen.
Typically, a composite solid propellant has a thermal expansion characteristic that is an order of magnitude larger than that of the enclosing or containing structure. A 100xc2x0 F. (56xc2x0 C.) change in operating temperature therefore may produce a propellant volume change of about 2%. Unless the configuration and support arrangement allow deformations to occur, thermal stresses in the propellant may cause fractures, undesired increases of burning surface area, and disasters upon ignition. Common provisions for thermal expansion include a central axial perforation for propellant grains bonded on their outer circumferential surfaces to cylindrical vessels and completely free outer surfaces for propellant grains bonded at either their forward or aft ends to vessel closure features.
The threat that gun accelerations may pose to the integrity of a propellant charge may be great unless care is exercised over the propellant configuration and means of supporting the propellant. Accelerations imposed within the gun tube upon gun-launched projectiles are hundredsxe2x80x94even thousandsxe2x80x94of times larger than those for rocket-launched projectiles. The tensile and shear strengths and elastic moduli of typical propellants are minuscule in comparison to the containing structure. For this reason, departures from a hydrostatic stress state during gun launch are accompanied by large deformations. At high forward acceleration, the propellant grain tends to completely fill the available volume of the aft end of the containing vessel.
During gun launch, alternatives to the aft end support arrangement for the propellant grain can be grave threats to the integrity of the propellant grain. Indeed, at acceleration levels typical of gun launches, neither the bonded circumferential surface of an axially perforated propellant grain nor an unperforated grain with a bonded forward end is stiff enough to eliminate the aft end support mode unless there is a great deal of empty space within the motor.
It follows that virtually the entire force that accelerates the propellant grain during gun launch is applied by direct bearing through its aft end. It also follows that the circumferential surface of the propellant grain will expand to fill the cylinder, imposing a radial pressure varying with depth (hydrostatically) from the aft end to the forward end.
Therefore, during gun launch, the case cylinder usually experiences tension in the hoop direction due to internal pressure applied by the propellant. This internal pressure may well be several times larger than the operating pressure later in flight when the propellant burns. Moreover, during gun launch, the axial force needed to accelerate the payload located forward of the rocket motor is carried around the propellant grain by axial compression in the rocket motor case, which should be proportioned so that buckling does not occur.
The buckling load for an axially compressed thin cylinder depends on its radius, thickness and length, and upon the modulus of elasticity at the actual imposed effective stress level. When the material xe2x80x9cyieldsxe2x80x9d, the modulus decreases from the initial value, Young""s modulus, to zero eventually (for ductile metals). Effectively, the material yields under the mixed tension and compression condition at a far lower stress level than if either stress were acting alone, and the modulus of elasticityxe2x80x94and the buckling loadxe2x80x94are thereafter much reduced. Thus, the thickness needed to assure a suitable safety factor is expected to be much higher than would be deduced for either the internal pressure or axial force alone.
In recent years, efforts to overcome the above-described behaviors of both the propellant and the case cylinder have turned to admitting the gun pressure to the interior of the rocket motor case. Examples of this approach are disclosed in U.S. Pat. No. 3,349,708, and are also explained in detail in U.S. Pat. No. 6,094,906. Admitting the gun pressure to the interior of the rocket motor with the fluid void-filler has both obvious and subtle implications. Among the obvious is that unless the exterior surface of the rocket motor is also exposed to gun pressure, the case cylinder may have to accommodate as much as 60,000 psi internal pressure, or morexe2x80x94an order of magnitude above the usual range of rocket motor operating pressures. To expose the external surface to pressure, an obturator, which is a sliding seal between the projectile and the gun tube that prevents the gun pressure from escaping around the projectile, is moved from the aft to the forward end of the rocket motor. It follows that, for the quasi-static situation at maximum acceleration, the differential pressure across the case cylinder wall is external pressure of varying magnitude, reflecting the hydrostatic gradient in the propellant grain. Further, the axial compression in the case cylinder disappears because the accelerating force for the payload is applied directly to the forward closure.
The subtle implications reflect the dynamic situations as the gun pressure rises rapidly upon ignition and as the gun pressure disappears when the obturator passes out of the gun bore. At the outside, because the orifice into the rocket motor is quite small, the intensity of the gun pressure applied to its interior lags the pressure intensity applied to the exterior. This threatens to buckle the case if the duration of the lag is large enough. Also when the obturator clears the gun bore, the small nozzle orifice prevents an instantaneous drop of internal pressure after the external pressure disappears. This threatens to burst the case unless it has been made thick enough to withstand the gun pressure levelxe2x80x94acting alonexe2x80x94that prevails immediately before the obturator clears the gun bore.
Given usual propellants and rocket motor nozzles, greater range or velocity is achieved for the projectile by configuring the rocket motor such that it can hold a maximum amount of propellant. However, the outside diameter, and hence the available volume for propellant, of gun-launched rocket motors is limited by the size of the gun bore from which the rocket motors are fired.
The volume of propellant in gun-launched rocket motors is maximized when the interior diameter of the rocket motor case cylinder is maximized by making the case cylinder as thin as possible. However, the case cylinder should be designed thick enough to withstand gun-launched loads and, when gun pressure is allowed within the case cylinder, the pressure differentials between the inside and outside of the case cylinder. The case cylinder should further be designed to withstand pressure differentials not only at maximum levels, but as the gun pressure rises early during launch and falls as the rocket motor exits the gun bore. Rocket motors designed according to the prior art must therefore survive gun launch loadings that are frequently far more severe than the later loadings during rocket motor burn. This may require thicker structures which diminish the volume available for propellant, and which increase the inert weight of the motor, thereby diminishing the attainable range or velocity of the projectile.
Thus, an advancement over the prior art would be achieved by introducing rocket motor configuration features that diminish the net loads that the rocket motor case cylinder must be designed to withstand during gun launch, thereby diminishing the inert weight and increasing the available propellant volume while also providing an insensitive munitions system that is effective against slow cook-off conditions.
Such rocket motor configuration features are disclosed and claimed herein.
This invention provides a rocket motor having an insensitive munitions system that is capable of passing a slow cook-off test.
This invention also addresses the above advancement by providing a gun-launched rocket motor designed to diminish the net loads that the rocket motor case experiences during gun launch, reduce the inert weight and increase the available propellant volume, and provide an insensitive munitions case and closure design.
This invention further provides a rocket motor design that accommodates size variations of the solid propellant as temperature conditions vary, such as while the rocket motor is being transported or stored, and incorporates in the insensitive munitions capability.
This invention still further provides a projectile having a rocket motor.
Several exemplary embodiments of the invention are disclosed and claimed herein.
In accordance with the present invention as embodied and broadly described therein, a rocket motor according to a first aspect of the invention has insensitive munitions capability. The rocket motor comprises a case including a cylindrical region, a closed forward end, and an aft assembly, the aft assembly comprising an aft closure member provided with an opening, the case being rupturable at an internal pressure burst level. The rocket motor further comprises a nozzle assembly coupled to the case, the nozzle assembly comprising a nozzle passageway. A primary propellant grain is contained in the case and has an auto-ignition temperature at which the primary propellant grain auto-ignites. The primary propellant grain is formulated to undergo thermal expansion in response to being heated to temperatures below the auto-ignition temperature so as to fill free volume, if any, inside the case and to apply an internal pressure to the case that is less than the internal pressure burst level. An igniter assembly is operational between an inactive state and an activated state. In the inactive state, the nozzle passageway is obstructed to substantially prevent the flow of combustion gases through the central nozzle passageway. In the activated state, the igniter assembly ignites the primary propellant grain and the nozzle passageway is substantially unobstructed to permit flow through the central nozzle passageway for propelling the rocket motor. An insensitive munitions charge is located inside the case and has an insensitive munitions auto-ignition temperature at which the insensitive munitions charge auto-ignites to release gas. The insensitive munitions auto-ignition temperature is below the primary propellant auto-ignition temperature. The insensitive munitions charge is present in an effective amount such that the gas released by auto-ignition of the insensitive munitions charge combines with the internal pressure applied by the thermal expansion of the primary propellant grain when the igniter assembly is in the inactive state to raise the internal pressure inside the case above the internal pressure burst level for rupturing the case before the primary propellant grain reaches the auto-ignition temperature thereof.
In accordance with the present invention as embodied and broadly described herein, a rocket motor according to a second aspect of the invention comprises a rupturable case including a cylindrical region, a closed forward end, and an aft assembly. The aft assembly comprises an aft closure member provided with an opening. A nozzle assembly is coupled to the case and comprises a nozzle passageway and a throat-barrier member for obstructing the nozzle passageway. A primary propellant grain is contained in the case, has an auto-ignition temperature, and is formulated to undergo thermal expansion in response to being heated within a range of temperatures below the auto-ignition temperature. As the primary propellant grain thermally expands, it will substantially fill free volume, if any, inside the case and to apply an internal pressure to the case that is less than the internal pressure burst level at which the case will rupture. An igniter assembly is positioned in the nozzle assembly and is operational between an inactive state and an activated state. In the inactive state, the igniter assembly is situated in the nozzle passageway and the throat-barrier member obstructs the nozzle passageway for substantially preventing the flow of gases through the nozzle passageway. In the activated state, the igniter assembly ignites the primary propellant grain, causing the nozzle passageway to become substantially unobstructed by the throat-barrier member and permitting the flow of gases through the nozzle passageway. The rocket motor of this aspect of the invention further comprises an insensitive munitions charge located inside the case and having an insensitive munitions auto-ignition temperature at which the insensitive munitions charge auto-ignites to release gas. The insensitive munitions auto-ignition temperature is below that of the primary propellant auto-ignition temperature. The insensitive munitions charge is present in an effective amount such that the gas released by auto-ignition of the insensitive munitions charge combines with the internal pressure applied by the thermal expansion of the primary propellant grain to raise the internal pressure inside the case above the internal pressure burst level. Because the igniter assembly is in the inactive state and the nozzle passageway is obstructed during insensitive munitions operation, the internal pressure cannot escape through the nozzle passageway and the internal pressure builds to rupture the case. Further, because the insensitive munitions auto-ignition temperature is below that of the primary propellant grain, rupturing of the case occurs before the primary propellant grain reaches its auto-ignition temperature. Thus, if the primary propellant grain is eventually ignited or auto-ignited, gases thereby generated will be able to escape the case through the ruptured portion of the case without producing significant propulsive forces.
In currently preferred embodiments, the first and second aspects of the invention provide a rocket motor having an insensitive munitions system that is capable of passing a slow cook-off test. Because there is no or substantially no free volume inside of the rocket motor and much of the pressure is produced by the expanding propellant grain, the insensitive munitions charge may be small and the quantity of gas it produces may be relatively small. Therefore, the case rupture resembles a hydroburst, not a gas burst.
In accordance with the present invention as embodied and broadly described herein, a rocket motor according to a third aspect of this invention comprises a case that is rupturable at an internal pressure burst level and includes a cylindrical region, a closed forward end, and an aft assembly. The aft assembly comprises an aft closure member provided with a central opening. A sliding piston is slidably retained within the aft assembly and the cylindrical region of the case so as to be movable from an at-rest position forward to a maximum pressure position in which the primary propellant grain is axially compressed to radially expand toward the cylindrical region of the case in response to firing of the rocket motor. The sliding piston is also slidable aftward in response to expansion of the primary propellant grain caused by elevated external temperatures. A nozzle assembly is slidably mounted within a central bore of the sliding piston to slide in tandem with the sliding piston. The nozzle assembly comprises a central nozzle passageway and a throat-barrier member for obstructing the central nozzle passageway. A primary propellant grain is contained in the case and has an auto-ignition temperature at which it auto-ignites. The primary propellant grain is formulated to undergo thermal expansion in response to external heat sources below the auto-ignition temperature. As the primary propellant grain thermally expands, it substantially fills free volume, if any, inside the case and applies an internal pressure to the case that is less than the internal pressure burst level. An igniter assembly is positioned within the nozzle assembly and operational between an inactive state and an activated state. In the inactive state, the igniter assembly is situated in the central nozzle passageway and the throat-barrier member obstructs the central nozzle passageway for substantially preventing flow through the central nozzle passageway. On the other hand, in the activated state the igniter assembly ignites the primary propellant grain and the central nozzle passageway is substantially unobstructed by the throat-barrier member to permit flow through the central nozzle passageway. An insensitive munitions charge is located inside the case and has an insensitive munitions auto-ignition temperature at which the insensitive munitions charge auto-ignites to release gas, the insensitive munitions auto-ignition temperature being below the primary propellant auto-ignition temperature. The insensitive munitions charge is present in an effective amount such that the gas released by auto-ignition of the insensitive munitions charge produces additional internal pressure inside the case. The internal pressure applied by the insensitive munitions charge combines with the internal pressure applied by the thermal expansion of the primary propellant grain to raise the total internal pressure inside the case above the internal pressure burst level. Because the igniter assembly is in the inactive state and the flow of gases through the central nozzle passageway is substantially prevented, the total internal pressure generated by the combination of the thermal expansion of the primary propellant grain and gases released by the insensitive munitions charge reaches the internal pressure burst level and causes the case to rupture. Case rupture occurs before the primary propellant grain reaches the auto-ignition temperature thereof. As a consequence, if the primary propellant grain eventually reaches its auto-ignition temperature or is otherwise ignited, gases produced by the primary propellant grain will be able to escape the case through the rupture in a relatively safe manner.
In accordance with this third aspect of the invention, the interior environment-controlled movable piston accommodates volume changes due to propellant thermal expansion and accommodates the substantial gun pressures associated with gun-launched projectiles. This enables a rocket motor structure design with the ability to withstand a dramatic rapid rise and dramatic sudden fall in pressure associated with gun-launched rockets. The movable piston also permits the rocket motor to be constructed from thinner and lighter materials to increase the available propellant volume and reduce overall inert weight. As a consequence, the range and effectiveness of the rocket motor are increased. Simultaneously, the rocket motor incorporates IM capability to permit the rocket motor to be rendered relatively harmless should the solid propellant inappropriately ignite while being stored or transported.
In accordance with a fourth aspect of this invention, the rocket motor comprises a primary insensitive munitions charge and a secondary insensitive munitions charge. The secondary insensitive munitions charge is formulated to have an auto-ignition temperature below the auto-ignition temperature of the propellant grain yet higher than the auto-ignition temperature of the primary insensitive munitions. The secondary insensitive munitions charge is preferably located in close proximity to the end burn surface of the primary propellant grain, so that auto-ignition of the secondary insensitive munitions charge in turn ignites the end burn surface of the primary propellant grain. As a consequence, the primary propellant grain will begin to burn from its end surface (where intended) before the primary propellant grain reaches its auto-ignition temperature. Thus, a significant portion or all of the primary propellant grain will be consumed by controlled burning at its end surface prior to auto-ignition of the primary propellant grain. Also, because the secondary insensitive munitions charge is designed to auto-ignite after the primary insensitive munitions charge, the case should already have burst (i.e., vented) by the time the secondary insensitive munitions charge auto-ignites.